Human immunodeficiency virus (HIV) is the etiological agent of AIDS. There are two types of HIV currently recognized, HIV-1 and HIV-2. HIV-1 is the predominant form worldwide. The form of HIV-1 that dominates the global epidemic is called the major group of HIV-1. There are three HIV-1 groups, the major group (M group), the outlier group (O group), and the non-M/non-O group (N group). The M group is further divided into at least eleven distinct genetic subtypes which are commonly referred to as clades, A, B, C, D, E, F, G. H, I, J, and K, with more sequences awaiting to be classified. Clade B is the most prevalent in the United States, while clade C is the most prevalent worldwide. Geographic distribution of genetic subtypes is continually changing, and current data offers incomplete estimates.
Approximately 95% of the new HIV infections are occurring in developing countries, thus a vaccine may be the most effective way to control the epidemic. However, developing effective vaccines to prevent HIV infection or neutralize HIV infection has been a difficult challenge to the scientific community. It is a primary goal to develop an HIV vaccine that can effectively elicit specific anti-viral neutralizing antibodies as well as cell-mediated immune responses to prevent infection and control the spread of HIV, with a potential for considerable breadth of reactivity across genetic clades. The extraordinary degree of genetic diversity of HIV has been problematic for vaccine development.